In recent years, telephone systems within offices and other organizations have increasingly made use of the concept of the private automatic branch exchange (PBX or PABX). Originally, the PBX handled only the communication of analog voice transmissions between a number of telephone extensions within an office, building or comparable environment and between those telephones and a lesser number of lines to a telephone company's central office. In recent years, there have also been PBX's which transmitted the voice signals in analog form, with analog-to-digital conversion (and digital-to-analog conversion) being handled directly at the telephone extensions. With the advent of computer technology has come the need to transmit digital information between personal computers and other data communications equipment co-located with telephones in similar types of environments. In an office, for example, an executive may have a computer terminal or other digital device in addition to a telephone. It is desirable in such environments to combine voice and data communications using wiring and equipment common to both types of transactions. This has led to the development of digital voice/data PABX equipment. One factor complicating the design of such equipment is the considerably different characteristics of voice signals in a telephone conversation and data signals within such systems. These data signals, of course, include not only those to be transmitted between data terminal equipment, but also those to be transmitted between different units of the PABX system.
Unfortunately, most digital voice/data PABX systems are designed optimally for networks of a particular size or particular limited range of sizes, in terms of the number of ports and telephones which may be accommodated. Outside the design range, the use of such a system may become uneconomical. To a growing company, this presents a dilemma: it can purchase a telephone system which is economical for its current size but which will become uneconomical as the company grows; or it can purchase a system larger than its current needs and uneconomical at that point but which will become economical as the company grows. Further, systems optimized for voice transmission may deal poorly with the handling of other data, and vice versa. Switching systems exist for controlling efficiently and economically small numbers of ports and other systems exist for handling large numbers of ports, but heretofore there has been no system available for addressing economically both (a) a digital PABX market spanning a range from just a few to up to about 960 ports and (b) voice and data transmission, including data for controlling the PABX system itself.
Recently, attention has been focused worldwide on all-digital telephones utilizing ISDN (integrated services digital network) technology. In this approach, analog-to-digital conversion, and vice versa, is made directly in the telephone and not in the connected switching system (whether public or private). A two-wire link is provided between the telephone and the switch, on which operates a pair of hydrid circuits with look-up table echo cancellation. Typically, data may be transmitted in each direction on this link at a combined rate of up to about 144 kbps; this consists of voice transmission at about 64 kbps, data transmission at about 64 kbps, and control information transmitted at about 16 kbps.
Of course, other transmission rates may be used. However, a pulse code modulated voice of eight bit companded quality at 8 kHz Nyquist sampling requires a 64 kbps channel. A user data channel capable of carrying asynchronous user stream of 56 kbps, as described in recent telephone company tariff filings, will also require a 64 kbps channel. Such transmission requirements imply either a four-wire link between the telephone and the switch, or else a two-wire link with circuitry at each end to permit sharing of the link between the two directions of transmission.
There are a number of disadvantages to this arrangement in a voice-data private switching system. Firstly, sometimes the link is not transparent to analog tone signals generated at the telephone during conversation; any such signals, which are used, for example, for remote control of voice mail systems, must in that case be recreated at the interface to the public network. Secondly, three channels serving different purposes must be mixed at one end of the link and separated at the other, implying circuitry for such operations. Thirdly, substantial bandwidth is dedicated to one user and cannot be shared with other users. Fourthly, the data bandwidth available at the telephone (data terminal) is limited to a maximum of 64 kbps. Fifthly, asynchronous data streams must be fed through a synchronous stream and extracted at the far end of the link. Sixthly, circuitry is required in the telephone itself for the conversion of voice signals from analog to digital form, and vice versa.
Another approach has used a voice switching system combining a star network for voice transmission and a loop network for transfer of control information, such as is described in French patent No. 2,538,662. In this known system, which utilizes the loop system described in French patent No. 2,127,876, a two level system of local and transit loops is employed; and computers used to control the common voice switch and routing and tarification translators are attached to the unique transit loop which interconnects all local loops by means of so-called "C-boxes". According to the French '662 patent, the port translation function, incorporating port and connection status information, is contained either in computers attached to each local loop or is contained in one of the port-serving computers on each local loop. This system provides service circuits (e.g., tone generators and tone receivers) on a common pool basis, with switched access to ports through the voice network. Although a plurality of access circuits are shown each with its own control computer, all connected to a common voice switch, all elements are apparently intended to be co-located; the system is not shown to be multi-nodal.
The system shown in those French patents is subject to several disadvantages. Firstly, use of a two level loop system (with relatively high level loops) entails further circuitry (including a C box) which would be unnecessary in small system applications of under 1000 ports, as in PBX's. Secondly, use of dedicated computers for network control and (rate and route) translation functions is wasteful of resources in small systems. Thirdly, switching access to service circuits is wasteful of voice network and control computer capacity and generates avoidable control messages adding to the traffic on the loop network. Fourthly, if data message switching is to be provided, such messages must be switched synchronously through the voice switch, which would entail asynchronous-to-synchronous conversion (and vice versa), and would be wasteful of bandwidth compared to an asynchronous packet switching approach. Fifthly, as stated above, the components of the system are apparently co-located and are not obviously distributable into different locations in a building.
It is therefore an object of the present invention to provide a system for voice and data transmission in a PABX environment, while substantially obviating the above-noted disadvantages of known systems.
It is a further object of the invention to provide such a system which is economical in use.
Yet another object of the invention is to provide such a system wherein a large range (i.e., from a low of about 30 to a high of at least 960 ports) may be accommodated economically, with system costs growing in a roughly linear relationship with the number of ports attached to the network.
Another object of the present invention is to provide a link from a voice/data telephone to a private switching system which eliminates the above-cited disadvantages of a link based on ISDN technology.